This invention relates to an aluminum nitride (AlN) substrate, more particularly to an AlN substrate, which is high in the bonding strength between the aluminum nitride (AlN) sintered product and the electroconductive metallized layer (hereinafter called "metallized layer") constituting the above substrate and also free from embrittlement through corrosion of titanium nitride (hereinafter called "TiN") constituting the metallized layer when bonding a metal member onto the metallized layer by soldering, etc.
In recent years, under the situation where high integration, high output and speed-up of electronic circuit are demanded, the substrate of semiconductor to be used therefor is required to have high thermal conductivity (heat dissipating characteristic) and electric insulating property, and further a coefficient of thermal expansion approximate to silicon chip. As the material satisfying such requirements, various ceramics have been widely used, and generally substrates of alumina (Al.sub.2 O.sub.3) or berylia (BeO) have been known. However, the Al.sub.2 O.sub.3 substrate has poor thermal conductivity, while the BeO substrate involves the problem of having toxicity. For these reasons, the substrate of AlN sintered product is recently attracting attention.
The AlN sintered product has a thermal conductivity which is as high as about 5-fold of that of Al.sub.2 O.sub.3, and is also excellent in electrical insulation, and yet exhibits a thermal expansion ratio approximate to silicon chip.
When such a substrate of AlN sintered product is used as the semiconductor substrate, it is necessary to mount a silicon chip on this substrate and further bond and mount metal members such as bonding wire, etc., by brazing, soldering, etc., but the above members cannot be directly bonded onto the AlN sintered product. For this reason, it is generally practiced to form an electroconductive metallized layer on the AlN substrate and bond the above members successively on the metallized layer.
As the method for forming a metallized layer on the surface of an AlN sintered product, there have heretofore been employed the direct bond copper method (DBC method) or the thick film method by use of copper, gold, silver-palladium. However, the metallized layer formed by these methods involves the following problems.
That is, according to the DBC method and the thick film method, a metallized layer is formed at a low temperature of about 600.degree. to 1100.degree. C. For this reason, under a high temperature, the bonded strength between the AlN sintered product and the metallized layer may be sometimes lowered, whereby the metallized layer may be peeled off. That is, the bonded strengh of the metallized layer to the AlN sintered product under high temperature is small. Therefore, when bonding a metal member through the metallized layer onto the substrate, it is difficult to practice soldering under high temperature in which silver solder, etc., is generally employed. Also, in the case when assembled in a semiconductor device, there may ensue the problem that the metallized layer may be peeled off from the AlN sintered product by the heat or the heat cycle generated during use, whereby reliability during use is lowered.
For such reasons, a metallized layer having molybdenum and TiN co-present on the surface of the AlN sintered product is now developing. In fact, this metallized layer has greater bonded strength under high temperature as compared with the layer formed by the DBC method or the thick film method of the prior art, but the following problems ensue when various members are applied by brazing, high temperature soldering on the metallized layer. That is, when various metal members are bonded onto the metallized layer, soldering is generally practiced in a reducing atmosphere containing hydrogen gas. However, when TiN is contained as the constituent component in the metallized layer, this TiN is corroded by hydrogen gas to be embrittled, whereby the bonded strength with the AlN sintered product will be ultimately lowered. Accordingly, when TiN is contained in the metallized layer, the metal member can be bonded with difficulty by soldering in a reducing atmosphere, or, even if bonded, since the metallized layer is embrittled, peel-off phenomenon will frequently occur during practical application to lower its reliability remarkably.